They've been around the block, maybe been married, maybe have a couple of kids no one knows about and given their life experience they're willing to risk it because they are "older and wiser" and "know" nothing will come of it.
I know some young girls who do BBFS with clients but for the most part when clients come clean about it to me (unsolicited) or when girls TELL me they've done it they tend to be 30+ (usually over 35) and very nonchalant about it.
When people go years without any repercussions for bad behavior it begets more irresponsible behavior and not because they're okay with catching a disease. We don't have Magic Johnson money - no one is catching HIV in this industry and living with it for 30+ years.
No, I think these girls just feel like after years of nothing happening they are cool with the risks. A baby can be eliminated, a disease isn't probable to them. Oh and I must say that an associate once said she's considered trying to get knocked up by a client for a guaranteed windfall.